I would like to point at two thinks:
1. In ArchiCAD GDL manual, it does say anything about how it can become a problematic issue working with .PNG or .Tif files type directly on the user interface dialogue box (i.e.: transparency only works in any other case, & I don’t know if that was also true for .SVG files)
SVG files are not used directly. They have to be converted to bitmap format – howto is put in my tread next to Yours.
PNG and TIFF files can have the transparent bit, so it can be used in AC UI, while JPG format do not have such feature, so the transparency will not work on it.